The present invention relates generally to improvements in digital wireless telephony. More particularly, the invention relates to advantageous aspects of receivers and repeaters for removing distortion from Baudot-encoded text telephony signals in a digital wireless telephony system.
Text telephony (TTY) is a technique developed to aid communication by persons who have difficulty using ordinary telephone equipment, typically because of difficulties with speech or hearing. Communication is accomplished by transmitting text messages over the telephone network, using special equipment to allow generation and transmission of text. The typical text telephony connection allows the choice of two-way text communication, or the use of voice communication by one party and text communication by the other.
TTY requires special terminal devices on both ends of the telephone line. One possible device is a teletype, which consists of a keyboard and a display, sometimes also including a printer. The teletype is connected to a telephone set via acoustic coupling or an RJ11 connector. Some teletypes are portable and lightweight, sometimes weighing less than a pound, and are well suited to work with mobile terminals. Another suitable device is an integrated teletype and telephone, which can perform both voice and text telephony. An additional existing device is a TTY modem which converts a personal computer (PC) into a TTY device. This modem serves as an interface between the PC and the telephone line, and allows the PC to operate as a TTY device. In particular, it performs encoding and decoding between ASCII character streams and TTY signals. A further device is an analog cellular phone adapter. This is a brand and model specific adapter which connects the RJ11 port on a teletype to a connection in a digital cellular phone.
In text telephony, the text messages are usually encoded using frequency shift key (FSK) modulation, and are transmitted over the telephone line and decoded at a receiver. TTY signals are transparent to the telephone network and are treated like any other audio signal.
Several coding schemes are available for use with TTY. The most common is Baudot coding, which is supported as a default by all or almost all TTY termninals. The Baudot code is a half duplex code using characters made up of bits represented by designated frequencies, each bit being 22.0xc2x10.4 msec long. A xe2x80x9c1xe2x80x9d (MARK) is represented by a frequency of 1400 Hz while a xe2x80x9c0xe2x80x9d (SPACE) is represented by a frequency of 1800 Hz. A transmitted character includes a start bit (0), five data bits, and one stop tone (1). The duration of the stop tone can be anywhere between a normal bit duration to two normal bits duration. If no other character is transmitted immediately after the current character, then the stop tone may be extended by a xe2x80x9cMARK hold timexe2x80x9d which may be anywhere from 0 to 300 msec.
The Baudot code supports two character sets, xe2x80x9clettersxe2x80x9d and xe2x80x9cfiguresxe2x80x9d. Figures include numerals and punctuation marks. One character set or the other is selected by depressing or releasing a shift key on a teletype keyboard. There are six characters common to both sets. These are BCKSPACE (0), L.FEED (3), SPACE (4), C.RET (8), FIGURES (27) and LETTERS (31). FIGURES and LETTERS are used only to switch between the two character sets. In addition, five character values in the figures character set are undefined, so that the total number of distinct characters is 51.
TTY techniques are well developed in the context of conventional wire telephony. Wireless telephony presents at least two significant issues which must be addressed if TTY is to be successful. These are the distortion to the TTY signals caused by the wireless channel, and the provision of an interface between the cellular phone and a TTY termninal. These issues have been satisfactorily dealt with in the case of analog phones. A number of manufacturers offer TTY interface gear for use between the cellular phone handset and an RJ11 port of a teletype unit. In addition, distortion caused by analog wireless communication is generally acceptable for transmission of Baudot coded messages.
Digital wireless communication provides numerous advantages for voice communication. However, prior-art digital wireless communication has not proven to be well suited for TTY communication, because channel distortion in digital wireless systems is sufficiently great that practical TTY communication is prevented. There are two causes for this problem. In commonly used speech coders used in IS-136 (which employs time division multiple access, or TDMA), source coding distortion is introduced into the Baudot coding process. The speech coders in question are IS-641 and VSELP. The coders used in IS-136 have a bit rate of 8 kb/sec, in contrast to the US-1 coder used in GSM-NA (PCS 1900) or the QCELP13 coder of IS95 (which employs code division multiple access, or CDMA), which provide bit rates of 12-13 kb/sec, and do not produce the source coding distortion problem. The IS-136 coder is sufficiently widely used that it will continue to be encountered as part of the communications infrastructure, and therefore any TTY communications system must be able to deal with source code distortions produced by IS-136 systems.
Additionally, channel distortion in code division multiple access (CDMA) communications systems introduces frame errors. A frame error is detected through the use of cyclic redundancy code (CRC), and when a frame error is detected, the frame is ignored and the previous frame is essentially repeated with somewhat diminished spectral peaks and amplitude. A frame error is likely to cause an error in the FSK process, which in turn leads to a character error. Since the normal frame error rate in the CDMA network is approximately 2% and there are approximately 8-9 frames in one Baudot character, this leads to an unacceptable character error rate.
The Federal Communications Commission has instructed the wireless telephony industry to develop a solution for TTY over wireless networks. In industry discussions, three types of solutions have been offered. A long-term solution might replace the Baudot coding by a more robust data communication protocol. A medium-term solution might retain Baudot coding, but change speech or channel coding standards to reduce degradation due to source and channel distortion. A short term solution would be one which would not require any change to industry standards.
A short term solution, one which would not require changes to standards and accompanying changes to infrastructure, has distinct advantages of simplicity and speed of implementation. Such a system could be designed to be compatible with presently existing equipment, and could be implemented by any manufacturer who wished to produce the necessary components. There would be no need for coordinated activity among manufacturers, which might delay the introduction of equipment due to the need to reach agreements among manufacturers.
There exists, therefore, a need in the art for a wireless TTY system which overcomes source and channel distortion, which is compatible with existing TTY devices, and which does not require industry standards changes in order to communicate successfully with other devices.
In one aspect, removal of distortion in TTY signals is accomplished by a repeater according to the present invention. A repeater according to the present invention is implemented in a wireless telephone receiver. A speech decoder processes input signals coming into the telephone receiver and produces decoder signals comprising a series of data frames, commonly referred to in the art as speech frames. The speech frames may consist of voice data or Baudot code data, depending on whether voice or Baudot code data is being received. The repeater receives the speech frames from the speech decoder, and determines whether the speech frames represent voice or Baudot data. If the speech frames represent Baudot data, the repeater interprets the speech frame data and produces as output a clean version of the same data, removing any corruption which may have been introduced by channel or other distortion. The input to the repeater consists of the decoded speech frames and a frame error indicator from the speech decoder. The output consists of a signal with distortion removed, as well as a detection flag which indicates whether or not the signal is a TTY signal.
The repeater is conveniently viewed as a connected Baudot receiver and a transmitter. The transmitter operates in a conventional manner, while the Baudot receiver detects the Baudot signals and removes channel distortion. The performance of the repeater is measured by two parameters: the correctness and quality of the generated TTY signals and the accuracy of the detection flag. The accuracy of the detection flag determines the ability of the repeater to reject non-TTY signals. It is also possible to employ the Baudot receiver without the transmitter, simply using the receiver to interpret the signals and drive a display device or a computer.